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Annual report pursuant to Section 13 and 15(d)

BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)

v3.6.0.2
BASIS OF PRESENTATION AND SIGNIFICANT ACCOUNTING POLICIES (Narrative) (Details)
T in Millions
12 Months Ended
Dec. 31, 2016
USD ($)
T
Dec. 31, 2015
USD ($)
T
Dec. 31, 2014
USD ($)
Related Party Transaction [Line Items] Ìý Ìý Ìý
Net Income (Loss) Attributable to Noncontrolling Interest $ 25,200,000 $ 900,000 $ (1,087,400,000)
LOSS ATTRIBUTABLE TO NONCONTROLLING INTEREST 0 7,700,000 1,113,300,000
Allowance for Doubtful Accounts Receivable 0 7,100,000 Ìý
Provision for Doubtful Accounts 0 7,100,000.0 0
Inventory Valuation Reserves 14,000,000 31,800,000 Ìý
Disposal of supplies inventory 17,400,000 Ìý Ìý
Tangible Asset Impairment Charges 0 0 537,800,000
Impairment of Intangible Assets (Excluding Goodwill) Ìý Ìý 13,800,000
Freight and venture partners' cost reimbursements $ 195,500,000 $ 180,900,000 278,000,000
Trading Day Window Determining Number of Common Shares Issuable on Conversion 20 Ìý Ìý
U.S. Iron Ore [Member] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Quantity Of Finished Goods | T 1.5 1.3 Ìý
Freight and venture partners' cost reimbursements $ 174,800,000 $ 157,300,000 271,000,000
Asia Pacific [Member] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Freight and venture partners' cost reimbursements $ 20,700,000 23,600,000 6,900,000
Empire [Member] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners 21.00% Ìý Ìý
Net Income (Loss) Attributable to Noncontrolling Interest Ìý (8,600,000) Ìý
Bloom Lake [Member] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Noncontrolling Interest, Ownership Percentage by Noncontrolling Owners 17.00% Ìý Ìý
Investments in Ventures [Member] | Hibbing [Member] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Ownership interest, equity method investment 23.00% Ìý Ìý
Other Noncurrent Liabilities [Member] | Hibbing [Member] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Equity Method Investments $ (8,700,000) (2,400,000) Ìý
Take or Pay Contracts [Member] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Deferred Revenue 3,400,000 Ìý $ 29,300,000
Customer Supplemental Payments [Member] | U.S. Iron Ore [Member] Ìý Ìý Ìý
Related Party Transaction [Line Items] Ìý Ìý Ìý
Deferred Revenue 77,100,000 89,900,000 Ìý
Deferred Revenue, Current 16,400,000 12,800,000 Ìý
Deferred Revenue, Noncurrent $ 64,200,000 $ 77,100,000 Ìý